Shaped structures of polyethylene terephthalate (sometimes abbreviated herein to PET), such as, for example, film and fibers, are well established commercial products. The film finds numerous applications, e.g., in packaging, photographic film base, and audio and video tape. PET films sometimes are difficult to handle, and especially to wind, because of the tendency of adjoining film layers to cling to one another. This problem can be solved, however, by adding to the film-forming material, prior to the extrusion of the film, certain fillers or slip additives, including certain crystalline sodium aluminosilicates, for example, as described in U.S. Pat. Nos. 3,876,608 to Anderson et al.
PET is commercially produced by a two-step process which comprises as the first step either ester interchange (or transesterification) between ethylene glycol and dimethyl terephthalate, preferably catalyzed by zinc acetate in the presence of a lithium compound such as, e.g., lithium glycolate, to produce di(2-hydroxyethyl) terephthalate (sometimes abbreviated herein as DHET), or direct esterification of terephthalic acid with ethylene glycol, which in the first step produces an oligomer having up to about ten repeating units. In the second step, either DHET or the oligomer, as the case may be, is polymerized to polyethylene terephthalate at an elevated temperature and at a reduced pressure. The polymerization step requires a catalyst, and a common catalyst for this step is antimony trioxide, as disclosed in U.S. Pat. No. 2,647,885 to Billica.
While antimony trioxide is an excellent catalyst, it nevertheless has various drawbacks, such as producing in the film defects such as black specks, large area polarization defects, and pimple rejects caused by polymer inclusions. Further, in other applications, for example, in making fibers, antimony trioxide, which often contains impurities, imparts undesirable color to the product. It also appears that a reaction between antimony trioxide and ethylene glycol occurs, producing a fairly volatile compound, which has a tendency to form deposits in the extruding equipment as well as in the spinnerets through which fibers are spun, so that they may affect the shape or the size of the fibers. Other, nonvolatile antimony compounds precipitate from the process stream and cause fouling of reaction vessels.
Finally, it is expected that with increasing concern about environmental problems, use of antimony trioxide may be limited or restricted in the future.
It, therefore, would be highly desirable to be able to produce the polymer in the presence of a catalyst which would not have all the above shortcomings.